add local password request info on doc

It may not be obvious that the local `/etc/exports` will need to be manipulated, and therefore might require root privileges. And that the "Password" prompt is about the local host user password, not the VM. (cf. #132)

docs/usage.md

@@ -114,6 +114,14 @@ Vagrant.configure("2") do |config|
 end
 ```
 
+An updated `/etc/exports` file will then be generated locally on the host, and this may require administrative (root) privileges. That's why you may have to type in your user's local password to edit the file: 
+
+```bash
+==> Preparing to edit /etc/exports. Administrator privileges will be required...
+Password: 
+==> Mounting NFS shared folders...
+```
+
 Please referer to http://docs.vagrantup.com/v2/synced-folders/basic_usage.html for
 more information about the supported parameters.